Modelling slab dehydration and water migration in subduction zones
From Institute for Energy Technology (IFE), Lillestrøm, Norway
The progressive release of water by metamorphic dehydration induces important physical-chemical processes, which have been invoked to explain many geophysical and petrological observations in subduction zones. These observations include: subduction-related volcanism, earthquakes, hydration of the mantle wedge, and rehydration of the oceanic slab. Yet, how dehydration and associated water migration progresses over time and with increasing depth has not been fully explored by numerical models. Here, we investigate this problem by incorporating the Perplex-based dehydration reaction into a thermo-mechanical modelling framework in which the released fluids are treated as porous fluids and migrate according to the two-phase flow. This framework allows us to investigate dehydration processes and fluid migration both in the oceanic slab and the mantle wedge, with the consideration of different subduction regimes. Two different types of hydrated rock are used in our models for the dehydration processes, with bulk compositions taken from Hacker (2008): mid-ocean ridge basalt (MORB), and depleted MORB mantle (hydrated slab mantle).
The effects of water source distribution, dehydration temperature, grain-size dependent permeability, subduction velocity, and weak back-arc lithosphere are explored. Our models show two high porosity planes in the slab, which are related to dehydration of oceanic crust and hydrated slab mantle at different depths. At the shallowest release depth, the 1st high porosity plane in the oceanic crust feeds water into the overlying lithosphere and up to the surface. Due to the tectonic under-pressure in the slab mantle, water from mantle dehydration migrates up-dip along the slab and form the 2nd high porosity plane to a much shallower depth than where it is released. This part of water finally feeds into the 1st plane at a certain depth. Our models further show that water can migrate into the surrounding mantle at different depths, under the condition that the pathway up to the surface is blocked or limited (e.g. grain-size dependent permeability and so on). Thus, three other directions of water migration are found: 1) downdip in the slab; 2) up into the deep mantle wedge; 3) down into the deep oceanic slab. These results are consistent with geophysical and petrological observations of subduction zones worldwide and have the potential to explain many fluid-related processes the subduction zone, such as slow earthquakes (i.e. slow slip events and episodic tremor and slip) and subduction volcanism.
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